DevTools: Add axe test for Changes drawer

This adds an axe test for the accessibility changes in #1659355. The
test is in a separate PR from the changes because of the flaky axe test
issue which should be fixed by #1638125.

Bug: 963183
Change-Id: Ibc3dbd8dd954e2493521bef48abdb65a802a91ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1666445
Commit-Queue: Jack Lynch <jalyn@microsoft.com>
Reviewed-by: Joel Einbinder <einbinder@chromium.org>
Auto-Submit: Jack Lynch <jalyn@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#677600}

third_party/blink/web_tests/http/tests/devtools/a11y-axe-core/changes/changes-a11y-test-expected.txt

+Tests accessibility in the Changes drawer.
+Loading files to compare.
+Showing the Changes drawer.
+Editor text content:
+.will-be-deleted-but-will-show-modified-because-first-line {
+    width: 10px;
+    height: 10px;
+}
+
+.stays-the-same {
+    font-size: 10px;
+    font-family: sans-serif;
+}
+
+.will-be-modified {
+    height: 100000%;
+.was-modified {
+    width: 0%;
+}
+
+.stays-the-same2 {
+    font-size: 10px;
+    font-family: Arial, Helvetica, sans-serif;
+}
+
+.was-added {
+    color: #00F;
+    color: #008;
+    color: #000;
+}
+
+.stays-the-same3 {
+    font-size: 15px;
+    font-family: Arial, Helvetica, sans-serif;
+}
+
+.will-lose-properties {
+    background: #000;
+    background: #400;
+    background: #800;
+    background: #B00;
+    background: #F00;
+}
+
+Running aXe on the Changes drawer.
+aXe violations: []
+
+

third_party/blink/web_tests/http/tests/devtools/a11y-axe-core/changes/changes-a11y-test.js

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+(async function() {
+  TestRunner.addResult('Tests accessibility in the Changes drawer.');
+  await TestRunner.loadModule('axe_core_test_runner');
+  await TestRunner.loadModule('changes');
+  await TestRunner.showPanel('sources');
+  await TestRunner.addStylesheetTag('../../changes/resources/before.css');
+  await TestRunner.addStylesheetTag('../../changes/resources/after.css');
+
+  TestRunner.addResult('Loading files to compare.');
+  const beforeSource = await TestRunner.waitForUISourceCode('before.css');
+  const afterSource = await TestRunner.waitForUISourceCode('after.css');
+  const content = await afterSource.requestContent();
+  beforeSource.setWorkingCopy(content);
+
+  TestRunner.addResult('Showing the Changes drawer.');
+  await UI.viewManager.showView('changes.changes');
+  const changesWidget = await UI.viewManager.view('changes.changes').widget();
+  await TestRunner.addSnifferPromise(changesWidget, '_renderDiffRows');
+  TestRunner.addResult(`Editor text content:\n${changesWidget._editor.text()}`);
+
+  TestRunner.addResult('Running aXe on the Changes drawer.');
+  await AxeCoreTestRunner.runValidation(changesWidget.contentElement);
+
+  TestRunner.completeTest();
+})();